The properties of photosynthetic reaction centers which make them interesting for technological applications are reviewed. Protein engineering has been used to systematically modify the oxidation potential of the initial electron donor in photosynthesis. The driving force and rates of electron transfer reactions in the reaction center have been altered by application of external electric fields. This leads to changes in the population of various intermediates which is reflected in changes in the absorption properties of the sample. The first example of engineering a new site for covalent attachment of new components to the reaction center is described.